1. Field of the Invention
This invention relates to a method of regenerating an aluminum surface cleaning agent which is used in an aluminum surface cleaning bath, and more particularly to stably and effectively regenerate the aluminum surface cleaning agent which is used to remove lubricating oil and aluminum powder (smut) from the surface of aluminum or aluminum alloy products.
2. Description of the Related Art
Products with aluminum surfaces, e. g., beverage containers made of aluminum or aluminum alloy, are ordinarily manufactured by a molding process known as "drawing and ironing" (hereinafter called "DI process"). During this DI process, lubricating oil is applied to outer surfaces of metal surfaces, and smut tends to adhere to inner surfaces of resulting containers. The surfaces of such containers are usually protected by surface treatment, conversion coating or painting, for example. Prior to the surface treatment or conversion coating, the foregoing lubricating oil and smut have to be removed from the metal surface. The aluminum surface is cleaned by the etching process. An acid cleaner is usually used for the surface cleaning so as to assure excellent surface treatment or conversion coating on the aluminum surface.
Conventionally, hydrofluoric acid cleaning agents are used as the acid cleaner as proposed in U.S. Pat. No. 3,728,188 and British Patent No. 1,454,974. These cleaning agents use chromic acid as an inhibiter so as to prevent corrosion of treatment apparatuses such as a surface cleaning bath or pump. However, the chromic acid and fluoride ions are so toxic that a special care should be taken with respect thereto to prevention of pollution of the working environment and disposal of used cleaning agents. Unfortunately, there is the problem that if the cleaning agent is free from the chromic acid, treatment apparatuses may be corroded. Further, if the fluoride ions are decreased, there is another problem that the cleaning agent suffers from lessened cleaning power.
In U.S. Pat. No. 4,728,456, a cleaner with a small quantity of or free from fluoride ions is proposed which can assure excellent cleaning power.
This cleaner contains 0.2-4 g/l ferric ions, but does not contain any chromium ion. The cleaner has its pH regulated to 0.6-2.0 with sulfuric acid and/or nitric acid. In the cited invention, the cleaner also contains 0.001-0.5 g/l fluoride ions. With this cleaner, it is considered that the etching of the aluminum surface by sulfuric acid and nitric acid is promoted by ferric ions (Fe.sup.3+). The promotion mechanism thereof is suspected to be due to a cathode reaction Fe.sup.3+ +e.sup.- .fwdarw.Fe.sup.2+.
The foregoing reaction consumes ferric ions in the cleaning bath. Therefore, it is necessary to replenish the ferric ions to the cleaning bath so as to restore and maintain the predetermined amount of the ferric ions. On the contrary, ferrous ions (Fe.sup.2+) will be gradually produced along with the cathode reaction of the ferric ions. The ferrous ions do not contribute to promotion of the etching. When the ferrous ions accumulate in large quantity, they produce a precipitate which makes the cleaning bath muddy, and reduces the cleaning power of the bath.
U.S. Pat. No. 4,851,148 proposes a method of solving the foregoing problems caused by generation and build-up of ferrous ions in the cleaning bath. Specifically, it is proposed to replenish aqueous iron compound solutions into the cleaning bath so as to compensate for consumed ferrous ions and an oxidizing agent so as to oxidize ferrous ion. Further, the amount of the ferric ions can be controlled in the cleaning bath by maintaining a predetermined oxidation reduction potential.
In the last mentioned invention, hydrogen peroxide is used as an oxidizing agent. However, when a strong hyrogen peroxide is supplied in the cleaning bath, the cleaning agent would splash. This is because an abrupt oxidation is caused by a small amount of metal salt mixed into the hydrogen perioxide.